Most of handy-type portable wireless units used in a gigahertz band are provided a whip antenna that is accommodated in the case, or a plane antenna such as a plate-form inverted F type antenna that is built in the case.
FIG. 1 shows a cellular phone, which is one of the conventional portable wireless units.
In a case 61, a circuit board 62 mounting the transmission and reception circuit (not shown) is accommodated, and numeric keys, function keys and liquid-crystal display (neither shown) are provided on the front side (backside of the case 61 in the drawing) of the case 61 formed by resin molding, and a whip antenna 63 (for transmission and reception), which can be retracted in the case 61, in flexible structure is disposed in the upper part of the case 61. A plate-form inverted F type antenna 64 is disposed in the vicinity of the backside wall of the case 61. The plate-form inverted F type antenna 64 comprises a radiation plate 64a and a short-circuit plate 64b that is formed in part of the radiation plate 64a and soldered to the earth land or shield plate of the circuit board 62. A power feed wire 65 is connected to a given position of the radiation plate 64a. 
As is generally known, the plate-form inverted F type antenna 64 is designed to satisfy an expression, L=W·λ/4, where the length of the radiation plate 64a is L, the width is W, and the wavelength is λ. It also has a characteristic that the bandwidth widens as the interval h of the radiation plate 64a and earth land or ground plate increases, and the efficiency (gain) lowers as the volume of the case (casing) decreases.
In the portable wireless unit in FIG. 1, the combination of the plate-form inverted F type antenna 64 and the whip antenna 63 composes a diversity antenna, thereby the multi-path fading can be decreased.
In the conventional portable wireless unit, however, if attempted to use a portable wireless unit accommodating the antenna (plane antenna) at a desired service frequency (hereinafter called desired frequency), the antenna needs to have a size according to the design value. Therefore, there occur problems that it becomes difficult to install its internal parts inside the case, it becomes difficult to get the sufficient frequency band, and the radiation efficiency (antenna efficiency) is likely to lower. In addition, there is a problem that the case space cannot be used efficiently.
To solve these problems, reported is, for example, Arai et al., “A Design of Uni-directional Potable Terminal Antenna and Effect of Phantom”, the general meeting of Institute of Electronics and Communication Engineers of Japan, SB-1-12 (1999).